Quote:
Originally Posted by rbinck
No OTA 1080p or 720i, btw. mpeg4 didn't exist as a standard when the ATSC was adopted.
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Partially true about 1080p, but the rest is accurate.
1080p at 24 fps and 30 fps are both ATSC formats that noone to my knowledge is using yet. Neither exceeds the bandwidth limitations of 6 MHz channels. Whether or not either is ever used by broadcasters remains to be seen. OTOH, 1080p at 60 fps requires about twice the available bandwidth and will in all likelihood will never be added to the ATSC spec because to do so would render millions of existing ATSC tuners out-of-date.
There is no 720i because it is incompatible with MPEG compression which requires the number of scan lines to be divisible by 16 for progressive scan and by 32 for interlaced scan. 720 is not divisble by 32.
One might also note that 1080 is not divisible by 16, but a trick is used to make both 1080i and 1080p compatible with MPEG. 8 blank lines (black pixels) are added to the bottom of the frame before compression takes place and they are simply discarded when the signal is decompressed to give the desired 16:9 aspect ratio. 1088 is divisible by 32 making it MPEG-compatible with both progressive and interlaced scan.
Incorporating MPEG-4 into the ATSC broadcast standard would also render millions of existing ATSC tuners out-of-date.